A call signal is a signal for informing a subscriber of reception of an incoming call when the call arrives. The call signal is also called as a ringer signal or a melody call signal. When the telephone set receives this call signal, a bell is rung in the telephone set. The call signal is generated in a telephone exchange equipment or some terminals and transmitted to the telephone set. Usually, the call signal conforms to the standard of each country. The call signal is a sinusoidal wave having a frequency of 10 to 70 Hz and an amplitude of around 200 Vp-p and it is transmitted during ON time and not transmitted during OFF time. For example, in Japan, the call signal whose frequency is 16 Hz, ON time is one second, and OFF time is two seconds is generated.
In a process for generating the call signal, a high voltage is generated by switching on and off a direct current low voltage that is usually safe for the human body even if the human body makes contact with it. However, there is a risk of electric shock when the human body comes into contact with the high voltage line in a process in which the call signal is generated because the generated voltage is a high voltage.
In contrast, in a call signal generating device described in patent document 1 (Japanese Patent Application Laid-Open No. 1990-257744), a high frequency signal of about 10 to 100 kHz from a high frequency generating circuit is inputted during an ON period, a switching component is driven by the high frequency signal to switch on and off a direct current power supply voltage Vcc via a step-up transformer, and whereby a high frequency voltage of about 75 V is generated on a secondary side of a transformer. Next, a positive component and a negative component are cut out by two diodes from the high frequency voltage that is an output of the transformer. The positive component output and the negative component output are periodically turned on and off, respectively and whereby a signal whose polarity is periodically reversed is generated. After that, the signal is smoothed and supplied to the telephone set as the call signal.
In the call signal generating device described in patent document 1, because the direct current power supply voltage is switched on and off at a high frequency of for example 100 kHz when the voltage is increased, the influence on the human body is small although the output voltage of the transformer is high. The signal whose polarity is periodically reversed is generated at a subsequent stage of the transformer. Accordingly, even if the human body comes into contact with arbitrarily two points, there is no case in which the human body comes into contact with the two points each of which has a positive polarity and a negative polarity. Therefore, the call signal generating device has high safety.